The invention relates in general to locks, in particular to combination locks, and specifically to such locks in which a dial drives a series of rotatable tumblers in one position and is moveable to another position for movement of the lock bolt cam.
Imperviousness to being picked is the essence of any lock. One method of picking a rotatable tumbler combination lock is to create a load on the tumblers. Many combination locks include a dial carrying out the clockwise and counter-clockwise rotations of the combination permutation and an external bolt cam lever for moving the bolt to unlock the lock. For such locks the combination permutation is carried out with the dial and the bolt cam lever manually operated to move the lock bolt. Other locks include an internal bolt cam biased (such as by a spring) to move into position for operating the lock bolt upon dialing of the combination permutation. So biased, the bolt cam applies a load to the lock tumblers. Such loaded tumbler locks are susceptible to picking by "feeling" the tumbler positions as the dial is turned, more specifically, by feeling the change in the load on the dial when a tumbler moves into its angular position for opening the lock.
Two approaches to frustrate picking a combination lock by the feel method are to include false gates in the tumblers and to hold the bolt cam away from the tumblers until after completion of the dialing permutation. The former is taught by the invention of Olaf Alfred Hage in his U.S. Pat. No. 2,104,516 which issued in January of 1938 and the latter by the invention of Peter J. Philips in his U.S. Pat. No. 3,981,167 which issued Sept. 21, 1976. The former seeks to make the load changes imperceptible and the latter to entirely eliminate any load change.
Combination locks are superior to keyed locks to the extent that keyed locks have a key hole which provides access to the interior of the lock and thereby makes the keyed locks susceptible to the lock picking technique in which the lock tumblers are set in lock opening positions by probing the tumblers with a mechanical pick inserted through the keyhole. However, generally in the past combination locks have been significantly bulkier in size than keyed locks. Because of their size, combination locks have not heretofore generally been used for the class of locks which has come to be known as vending machine locks, typically an Ace.TM. or Gem.TM. lock having a 3/4 inch diameter cylinder with 5/8 inch across flats.
A general object of the invention is to provide a lock which is difficult to pick.
Another object of the invention is to provide a lock which minimizes if not totally eliminates load changes on the lock tumblers which can be felt during turning of the dial.
An additional object of the invention is a combination lock which is highly compact.
A further object of the invention is to provide a lock which is inexpensive to manufacture and easy to assemble.